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Cocaine is making a comeback in the US. It appears to be a case of supply driving demand.

After years of falling output, the size of Colombia's illegal coca crop has exploded since 2013, and the boom is starting to appear on US streets.

"There are troubling early signs that cocaine use and availability is on the rise in the US for the first time in nearly a decade," the State Department noted last week in its annual report on the global narcotics trade.

According to test samples of the drug seized on the streets, 90 per cent of the cocaine for sale in the US is of Colombian origin, the report said.

The number of overdose deaths in the US involving cocaine in 2015 was the highest since 2006 and the second-highest since 1999, the federal Substance Abuse and Mental Health Services Administration reported.

And the number of young Americans who admitted to trying cocaine for the first time increased a whopping 61 per cent from 2013 to 2015, the most recent National Survey on Drug Use and Health found. According to that survey, 1 in 20 American adults ages 18 to 25 used the drug in 2015, with the highest percentage concentrated in the Northeast. In New Hampshire, more than 10 per cent of young adults used cocaine in 2015.

This surge in consumption can be traced directly to Colombia's bumper harvest. The country's illegal coca crop doubled between 2013 and 2015, reaching nearly 162,000ha. That's almost twice as much as the combined output of Peru and Bolivia, the world's second- and third-largest producers.

Cocaine trafficking from Colombia is at "record levels," the State Department said, warning that even bigger loads are probably on the way.

Since 2000, the US has sent Bogota more than US$10 billion ($14 billion) in counternarcotics and security assistance. That congressional funding is widely credited with helping tip Colombia's 52-year civil conflict in the Government's favour and forcing the Farc rebels to the bargaining table. A peace plan was signed in November.

The State Department report cites four major reasons for the sudden coca-growing binge by Colombian farmers. The first is that Farc rebels appear to have encouraged farmers in areas under their control to plant as much coca as possible in preparation for the end of the war, "purportedly motivated by the belief that ... investment and subsidies will focus on regions with the greatest quantities of coca".

At the same time, the Government reduced eradication in those areas "to lower the risk of armed conflict". The Government also ended aerial spraying with herbicides in favour of manual eradication. But local farmers have blocked roads and found other ways to thwart the removal.

The final factor is the Government's financial squeeze, "resulting in a 90 per cent reduction in the number of manual eradicators in 2016 as compared to 2008".